The term "heat-sensitive recording paper" means a recording paper of the kind which forms an image by physical or chemical changes of some of its constituents caused by applying thermal energy. A large number of such processes have been studied.
There are a number of known processes for utilizing a physical change caused in a certain substance by the application of heat, an example of which is the so-called wax type heat-sensitive recording sheet. These sheets are used for recording electrocardiograms and so on. There are also a number of known processes which utilize chemical changes caused by heat. For instance, one process takes advantage of a particular compound which undergoes coloration upon the application of heat. Another process takes advantage of a particular combination of substances which can react with one another under applied heat and form a color. Specific examples of combinations which can effect a color reaction when heat is applied include combinations of ferric salts of higher fatty acids such as stearic acid or the like with polyhydroxy aromatic compounds as described in U.S. Pat. Nos. 2,663,654, 2,663,655, 2,663,656, 2,663,657, and so on, those comprised of starting materials for producing dyes such as azo dyes or oxazine dyes as described in Japanese Patent Publication No. 9,240/63 and so on, and those comprised of colorless dyes such as Crystal Violet lactone and the like with phenolic compounds as described in U.S. Pat. No. 3,539,375, and so on.
All of these heat-sensitive recording papers have color reactions which obey a first-order kinetics. Therefore, a developing step is not required. Such being the case, these heat-sensitive papers are desirable because the recording apparatus therefor can be made light weight and miniaturized. These advantages have recently caused a rapid increase in the demand for using these heat-sensitive recording papers. However, these heat-sensitive recording papers are not desirable because of their slow response speeds. In order to carry out high speed recording, a large amount of heat energy must be applied on the heat-sensitive recording paper in a short time because the amount of heat energy per unit area necessary for recording is constant. However, the recording element has a limited recording energy depending on the power of source.
Various means have been devised to eliminate these defects by altering the recording apparatus and/or the recording paper. A typical example thereof is the improvement upon the surface smoothness of the heat-sensitive recording paper as described in Japanese Patent Publication No. 20142/77, and Japanese Patent Application (OPI) No. 47351/73 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). The smoothness of a heat-sensitive recording paper has, in general, been improved by subjecting the recording paper to a surface treatment such as super calendering or the like. The improvement in the surface smoothness increases the heat transfer efficiency between the recording element and heat-sensitive recording paper and contributes to a gain in the response speed of the paper.
However, such surface treatment cannot make the recording paper smooth without various disadvantages. One disadvantage is the occurrence of fog, that is, the color reaction which takes place in the course of the surface treatment resulting in undesired coloration in the recording paper. A proposed means of eliminating this disadvantage is the addition of grain-shaped wax. This was proposed in Japanese Patent Publication No. 14531/75 and U.S. Pat. Nos. 4,032,690 and 3,445,261. However, waxes usually have large heat capacities and require great heat upon fusion. Consequently, they retard the thermal response of the heat-sensitive recording papers.
Another disadvantage is a lowered facility with respect to writting with a pencil, a ball-point pen or the like. The reason for this disadvantage is obvious considering that such writting means functions through friction with paper.
Yet another disadvantage relates to a production problem. More specifically, the smoothening step is carried out using a super calender or the like operating independently of the main machine for producing the recording paper. Therefore, production efficiency is remarkably lowered.
Still another disadvantage is impediment to running of a recording paper on the recording element. More specifically, the recording paper sticks to the recording element at colored parts because the density of the heat-sensitive color-forming layer is greatly increased by the smoothening treatment. This causes the diffusion of heat fusable substance at the time of recording to be hindered in the direction of the interior of the coated layer.
However, because improved smoothness contributes to elevation of recording speed to a comparatively large extent such surface treatment is carried out at present, notwithstanding the various disadvantages described above.